Spatial light modulators, also referred to as light valves, have found use in many different fields. One particular industrial field in which these devices have been employed is the display industry. Another field where spatial light modulators have made a significant impact is the printing industry, where they are extensively used with lasers to image various recording media. Recording media can include various printing plates, printing sleeves, and printing cylinders for example. The lasers employed in these applications often emit radiation having wavelengths suitable for marking a sensitized surface of the recording media. In some cases the lasers emit radiation comprising near-infrared or ultraviolet wavelengths.
Spatial light modulators typically include a one or two-dimensional array of light valve channels. Each of the channels can be selectively operated to provide an output radiation beam which can be used to form a unit element of an image typically referred to as an image pixel. In some cases, an output radiation beam is provided by reflecting radiation from the spatial light modulator. In some cases, an output radiation beam is provided by transmitting radiation through a spatial light modulator.
One particular subset of spatial light modulators is based on the reflection of incident radiation from micro-miniature deformable mirrors. Prior art deformable mirror light modulators can be generally divided into several types. For example, cantilever or hinged mirror light modulators deflect radiation when bending or tilting the mirror elements. A well-known example in this category is the digital micro-mirror device (DMD) technology developed by Texas Instruments Incorporated. Membrane light modulators employ a flat membrane that is deformed into a concave or spherical mirror which focuses radiation.
Another subset of spatial light valves diffracts radiation by forming a periodic physical pattern. A well-known example in this category is the grating light valve developed by the Silicon Light Machines Corporation of Sunnyvale, Calif. Total internal reflection (TIR) spatial light modulators include an electro-optic material whose optical properties change in accordance with the strength of an electric field established within the material. Conventional TIR modulators typically include a plurality of electrodes that are arranged in an interdigitated relationship on a support surface of an electro-optic member. Other surfaces of the member are arranged to cause input radiation to refract and undergo total internal reflection at the support surface. Upon the application of a suitable voltage to a corresponding one of the electrode sets, an electric field is established in a portion of the electro-optic member which alters the refractive index of the member and causes the electrode set to behave in a manner similar to a diffraction grating.
Spatial light modulators can require calibration for various reasons. For example, in imaging applications calibration may be required to alleviate image artifacts. Typically, there are a number of imaging parameters that need to be optimally set to achieve a desired quality result. One important parameter is the level of radiation exposure provided on the recording media. Exposure is typically defined as the amount of radiant energy per unit area that impinges on the recording media during the imaging process. Depending on the recording media, it may be necessary to control this parameter to within a few percent or less. This situation is further compounded when multiple output radiation beams are provided by a spatial light modulator. In this case, each beam needs to impart a substantially equal exposure on the recording media so that various artifacts including banding are not created.
Calibration of spatial light modulators can include balancing the various radiation beam intensities provided by an array of modulator channels in a process typically referred to as beam balancing. Beam balancing techniques attempt to establish a desired intensity distribution (i.e. also referred to as intensity profile) across all the output radiation beams that can be provided by the channels of a spatial light modulator. To achieve a desired intensity profile, one needs to know with some degree of certainty how each image pixel changes in response to change in the control settings of a channel corresponding to the image pixel and possibly, channels that neighbor the corresponding channel.
Some conventional beam balancing methods have employed multi-value detectors to measure an intensity profile. Some conventional multi-value detectors typically include a plurality of detection elements whose number equal, or exceed the number of spatial modulator channels that are activated to provide the detected output radiation beams. Radiation from each of a plurality of different sets of the modulator channels can be simultaneously detected by multi-value detectors to provide a spatial distribution of intensity values, each of the intensity values corresponding to the radiation provided by a different one of the modulator channel sets. In the limit, multi-value detectors can be employed to determine an intensity profile across entirety of the modulator array on the basis of single channel resolutions. Examples of multi-value detectors include laser beam profilers that are diagnostic devices that can measure the entirety of an intensity profile of a supplied radiation. Beam profilers can be used to accurately determine a detailed intensity profile shape of a plurality of radiation beams. Beam profilers can include photo-sensor based beam profilers that comprise visible or near-infrared CCD or CMOS sensors. Beam profilers can include scanning beam profilers that scan a beam profile with various pinholes, slits, or knife edges.
Despite their accuracy and resolution, many multi-value detectors can be considered to be prohibitively expensive if they are to be incorporated into a recording apparatus. To alleviate these costs issues, the use of single-value detectors has been proposed for use in the detection of output radiation beam intensity. Single-value detectors are simpler, less complicated, and less expensive than multi-level detectors. In a similar fashion to multi-value detectors, single-value detectors can simultaneously detect radiation from each of a plurality of modulator channels. However, single-value detectors can not distinguish between the different portions of the radiation provided by each of the modulator channels. Consequently, single-value detectors provide only a single intensity value representing the total radiation that is provided. The data determined by using a single-value detector does not contain any information on how the radiation intensity is spatially distributed. In particular, it does not indicate how much energy each image pixel would receive during exposure.
To overcome this shortcoming, single-value detectors can be employed to provide an intensity profile for all the operable channels in a spatial light modulator by dividing all the modulator channels into sets and individually activating each set to provide corresponding radiation which is separately measured by the detector. An intensity value is separately determined for each of the channel sets and an intensity profile is generated by mapping each of the separately determined intensity values with positional information of a corresponding one of the channel sets. For example, a portion of the intensity profile can be generated by measuring the total intensity of radiation provided by a first one of the channel sets while the remaining channels are turned off. Repeating this measurement for each of a sequence of different channels sets making up the remainder of the spatial light modulator provides a set of intensity values representing the intensity profile.
The number of channels employed in each of the channels sets during this process is typically based on several factors. For example, channel sets comprising only a few channels each can provide a suitable granularity for making effective corrections to intensity deviations highlighted by a subsequently determined intensity profile. However, larger numbers of these channel sets having fewer channels would be required to complete the intensity profile thereby increasing the calibration time. The present inventors have additionally determined that the number of channels employed in each channel set also has an effect on the accuracy of the intensity value measured by a single-value detector. For example, FIG. 1 shows a plot comparing various intensity profiles for a recording head produced by the Eastman Kodak Company. In this particular case, the recording head employs a spatial light modulator having 896 channels. Three different intensity profiles are illustrated in accordance with the KEY of the FIG. 1 plot. The various intensity values are shown in arbitrary units. A multi-value detector intensity profile 450 acts as a base-line to compare the accuracy of a first single-value detector intensity profile 460 and a second single-value detector intensity profile 470. Each of the multi-value detector intensity profile 450, the first single-value detector intensity profile 460 and the second single-value detector intensity profile 470 have been “smoothed” for clarity and therefore do not show scatter among individual intensity value data points that each of the profiles was generated from.
Multi-value detector intensity profile 450 represents a condition where each of the channels in the array has been balanced using a multi-value detector (i.e. a laser beam profiler) in a manner similar to that previously described. In this case, the intensity level of various channels was determined using the multi-value detector, and control levels of each of the channels were adjusted to balance the channels to produce the substantially level multi-value detector intensity profile 450. Each of the first single-value detector intensity profile 460 and the second single-value detector intensity 470 were generated with the use of a single-value detector. Specifically, after the spatial light modulator channels were balanced using the multi-value detector, the intensities of different sets of the balanced channels were measured using the single-value detector in a manner similar to that previously described. A plurality of first channel sets, each comprising thirty two (32) channels was used to generate the first single-value detector intensity profile 460 while a plurality of second channel sets, each comprising three (3) channels was used to generate the second single-value detector intensity profile 470.
The FIG. 1 plot shows that despite having accurately balanced the spatial light modulator channels using a multi-value detector, each of the first single-value detector intensity profile 460 and the second single-value detector intensity profile 470 show deviations from this balanced condition. In this regard, each of the first single-value detector intensity profile 460 and the second single-value detector intensity profile 470 is distorted. The first single-value detector intensity profile 460 that was generated using the first channel sets comprising thirty two (32) channels is shown deviating by about 1% from the uniform multi-value detector intensity profile 450 while the second single-value detector intensity profile 470 that was generated using second channel sets comprising three (3) channels shows as much as 4% deviation. Although they do not wish to be bound by any particular theory, the present inventor believes that due to the details of the operation of the spatial light modulator and the propagation of the radiation in the recording head, an intensity profile generated using a single-value detector will typically deviate from an intensity profile generated with a multi-value detector. The deviation magnitude depends on the number of channels in the detected channel sets, with stronger deviations resulting from channel sets having fewer numbers of channels.
There is a need to provide improved methods and systems for calibrating a spatial light modulator. There is a further need to provide improved methods and systems for reducing deviations in an intensity profile generated for a spatial light modulator using a single-value detector.